Recent applications of engines have increased the importance of their durability, particularly where an engine must be run for a long time period. For example, internal combustion engines are now often used in drones or unmanned reconnaissance aircraft, which are required to continuously operate for well over 24 hours. This increases problems associated with the durability of the engine. It is also desirable to provide an engine that effectively operates on a variety of fuels, including gasoline, diesel fuel and heavy fuel. In addition, it is desirable to improve the combustion efficiencies of two stroke engines generally to obtain improved fuel economy and reduce emissions.
To accomplish these ends several approaches have been used in the past. Improved engine combustion is known to be improved by applying insulative ceramic layers to the combustion chamber surfaces, as disclosed for example, in U.S. Pat. No. 4,852,542 or 5,820,976. While this improves combustion by allowing the engine to run hotter, the increased heat also causes the engine components to breakdown more quickly. In addition, this approach increases the temperature of the air intake, which alters the appropriate air-fuel ratio and volumetric efficiency. While this might be addressed by adding a turbo charger or fuel injection system to the engine, such a system increases the size, weight and cost of the engine. Moreover, turbochargers generally increase engine wear because they require the engine to operate at even higher temperatures and pressures. Accordingly, providing additional insulating capacity to the combustion chamber surfaces is not suitable for engines that must be continuously operated for long time periods. Moreover, adding conventional turbo charging or fuel injection systems can increase the weight, size and cost of an engine to an unacceptable level.
It is also desirable to improve reduce toxic emissions from engines. While 2-stroke engines are generally more efficient than 4-stroke engines, they have very poor air handling or breathing characteristics unless they are fuel injected or turbocharged because of the natural intake and exhaust port timing characteristics of 2-stroke engines. When the air fuel (A/F) charge is directed into the combustion chamber, a small fraction of the exhaust port is still open. Because the air fuel charge of a 2-stroke engine is not mixed well it does not immediately burn. This combustion delay coupled with partially open exhaust port allows passage of some of the air fuel charge to escape from the open area exhaust port. In some cases, the amount of unburned fuel escaping to the exhaust train can be up to 60% of the original fuel charge. This pollutes the environment and wastes fuel. Accordingly, many manufacturers of, for example, outdoor power tools, are replacing the 2-stroke engines in their equipment with 4-stroke engines so as to provide a more environmentally friendly product.
Accordingly, it is desirable to provide an engine that can generate more power, uses less fuel, weighs less, costs less, and has improved emissions characteristics.